SmI(HO) Reduction of Electron Rich Enamines by Proton-Coupled Electron Transfer.

Samarium diiodide in the presence of water and THF (SmI(HO)) has in recent years become a versatile and useful reagent, mainly for reducing carbonyl-type substrates. This work reports the reduction of several enamines by SmI(HO). Mechanistic experiments implicate a concerted proton-coupled electron transfer (PCET) pathway, based on various pieces of evidence against initial outer-sphere electron transfer, proton transfer, or substrate coordination. A thermochemical analysis indicates that the C-H bond formed in the rate-determining step has a bond dissociation free energy (BDFE) of ∼32 kcal mol. The O-H BDFE of the samarium aquo ion is estimated to be 26 kcal mol, which is among the weakest known X-H bonds of stable reagents. Thus, SmI(HO) should be able to form very weak C-H bonds. The reduction of these highly electron rich substrates by SmI(HO) shows that this reagent is a very strong hydrogen atom donor as well as an outer-sphere reductant.